Due to the increasing demands on the agricultural industry, there is a commensurate need for supplementing plant nutrients, either by soil or foliar application. These nutrients include a variety of minerals such as phosphorus, zinc, iron, copper, magnesium, manganese, molybdenum and boron. Elemental sulfur supplies soluble sulfate. It has the further advantage of reducing soil alkalinity.
It is often desirable to supplement soil concentrations of more than one of these nutrients in a single application. Thus it would be beneficial to have a single nutrient combination in an easily handleable form that would not segregate during transport or application.
Obviously, all of the nutrients could be supplied individually; and that procedure has the equally obvious advantage of allowing immediate on-site variation of nutrient concentration. However, it has the disadvantage that all nutrients are not maintained in immediate proximity to each other. In some cases this factor is not particularly significant. However, I have found that bacterial sulfur oxidation creates an acidic environment in the vicinity of the sulfur particles and that this environment can convert insoluble mineral nutrients such as mineral oxides, carbonates and sulfides, to soluble sulfates. This acidification even improves the mobility of nutrients applied as soluble compounds in calcareous soils by reducing the tendency of otherwise mobile compounds to convert immobile hydroxides, oxides, or the like. These nutrients must be made available to the plant roots in a soluble, mobile form to allow their assimilation by the crop.
The acidizing effect of elemental sulfur applied in any reasonable dosages, e.g., 20 to 800 pounds per acre, exists only, or at least to a large extent, in the area adjacent the sulfur particle. Thus, at least in calcareous soils, soil pH will increase with distance from the particle surface. Due to this effect and the beneficial influence of sulfur acidification on nutrient mobility, particularly on the conversion of insoluble, immobile compounds to mobile forms, it would be desirable to assure that all of the applied nutrient is fixed in the immediate vicinity of, and preferably within, the sulfur particle. It is even more desirable that the nutrient compound be evenly distributed throughout the matrix of the sulfur particle to assure gradual nutrient release rather than a slugging effect that would result from alternative procedures such as surface coating.
There are a number of mineral nutrient sources. Some are soluble such as the sulfates, nitrates and complexes with chelating agents, all of which are known in the agricultural industry. Inexpensive nutrients can be obtained as the oxides, sulfides and carbonates, the oxides being particularly preferred due to availability and low cost. However, in an attempt to form homogeneous combinations of these supplements with molten sulfur I discovered that they could not be distributed throughout the sulfur melt even with reasonably severe agitation. While better distribution, if not homogeneous combinations, might be obtained with extremely high shear mixing techniques in some cases, such techniques increase capital cost and operating expense.
Therefore, it is one object of this invention to provide a method for producing homogeneous, solid fusions of inexpensive, finely divided mineral nutrients in a continuous rhombic sulfur matrix. Another object is the provision of an improved continuous process for the formation of such homogeneous solid fusions. Yet another object is the provision of improved, homogeneous solid fusions of inexpensive mineral nutrients dispersed throughout a continuous rhombic sulfur matrix which assure an even, gradual nutrient release from the sulfur particles as a result of gradual, bacterial oxidation. Another object is the provision of methods for producing such mineral-sulfur solid fusions that result in little or no nutrient loss during manufacture, e.g., by leaching by aqueous quench media.
In accordance with one embodiment, homogeneous solid fusions of particulate mineral nutrients dispersed in a continuous rhombic sulfur matrix are obtained by dispersing throughout a sulfur melt a nutrient-hydrocarbon comixture containing at least about 10 weight percent of a finely divided water-insoluble plant nutrient compound and at least about 2 weight percent of a non-polar hydrocarbon. The hydrocarbon concentration must be at least sufficient to completely wet the exterior surfaces of the finely divided nutrient. Mild agitation is sufficient to assure even distribution of the nutrient-hydrocarbon comixture throughout the melt. The resulting suspension is then cooled to a temperature below the melting point to form blocks or particles of the homogeneous, nutrient-sulfur fusion as desired.
The nutrient compounds are selected for their relatively low cost, availability and water insolubility. Water insolubility is particularly desirable in the preferred embodiment which involves comminuting and solidifying the sulfur-nutrient melt by contacting with water. Such nutrients include calcium phosphate and the oxides, sulfides and carbonates of zinc, iron, copper, magnesium, manganese, molybdenum and boron. Of the nutrients other than phosphorus, the oxides are particularly preferred due to their very low solubility, availability and low cost.
To one degree or another, all of these materials suffer from the disadvantage that they can be dispersed in molten sulfur only with considerable difficulty, if at all. Thus the formation of a completely homogeneous distribution of the nutrient compound in the sulfur melt prior to quenching is difficult if not impossible. I have found that this difficulty can be overcome by assuring that all surfaces of the nutrient compound are coated with a non-polar hydrocarbon prior to admixture with the sulfur melt.
The same is true of the insoluble calcium phosphate source. By these methods the phosphate can be easily and uniformly dispersed in molten sulfur in relatively high concentrations, e.g., up to about 50 weight percent. The combinations also have other notable advantages in agricultural use.
Calcium phosphates are widely available as several minerals such as apatite, usually containing fluorine, e.g., fluoro-apatite, some copyrolites and some phosphorites of which significant deposits exist in both this and foreign countries.
Available, i.e., soluble phosphate, is usually derived from insoluble calcium phosphate by dissolving the calcium phosphate in sulfuric acid to release phosphoric acid and calcium sulfate (gypsum) both of which have agronomic value. The product acid, generally known as wet-process phosphoric acid, is a widely used industrial and agricultural commodity. The usual manufacturing methods require the preliminary manufacture of sulfuric acid and, in the great majority, if not all cases, require separation of the dilute phosphoric acid from insoluble calcium sulfate byproduct.
The most widely used sulfuric acid manufacturing procedure involves sulfur oxidation to sulfur dioxide which is then converted to sulfur trioxide over a vanadium catalyst followed by hydrolysis to the acid. Once obtained, this acid is used to convert insoluble calcium phosphate (from fluoroapatite or otherwise) to insoluble calcium sulfate and dilute phosphoric acid. The phosphoric acid is then recovered in the supernatent phase and concentrated by conventional procedures. Calcium sulfate is recovered by filtration. Both of these materials can be used as soil additives.
I have found that the embodiment of this invention employing crude phosphate rock eliminates all of the manufacturing steps involved in the conventional production of calcium sulfate and phosphoric acid referred to above. It does so in a very effective manner in that the resulting sulfur fusions, being homogeneous, assure continuous availability of soluble phosphate and sulfur as calcium sulfate at a rate dependent only on the rate of sulfur-active bacterial action.
In this respect another advantage of this invention is that the activity of heterotropic thiobacillae is increased due to the presence of hydrocarbon in the sulfur matrix. The hydrocarbon provides a carbon source for the bacteria thereby accelerating their growth and consequently the conversion of sulfur to sulfuric acid. By this technique the amount and type of hydrocarbon can be controlled or selected to control the rate of phosphate and sulfur release. All of these things are accomplished with very inexpensive, readily available minerals, e.g., elemental sulfur and crude phosphate rock.
Accordingly, the methods and compositions of this invention have several significant advantages over those known to the prior art. They allow for the use of inexpensive readily available water-insoluble oxides, sulfides and carbonates. They require much less agitation or shear to obtain adequate nutrient distribution than is the case with alternative methods. They result in easily obtained, homogeneous, macronutrient dispersions in both the sulfur melt and product matrix. They minimize or completely eliminate nutrient loss during formulation or shipment, and they do not so weaken the product particles as to make them friable in transport or use.
On the contrary, I have found that the hydrocarbon results in several advantages other than nutrient dispersibility. The product particles, when formed as such, are more fluid than are particles not containing hydrocarbon. They have significantly less tendency to dust or autoignite during transport or use and they do not bridge and plug transportation and application equipment. Product homogeneity assures more uniform nutrient release. The hydrocarbon also accelerates heterotrophic bacteria activity and therefore increases nutrient release rate. Due to the low shear mixing made possible by these methods, they are readily adaptable to continuous, low shear formulation procedures.
The preferred metal nutrients are zinc, iron, copper, magnesium, manganese, molybdenum and boron in the form of the corresponding oxides, carbonates and sulfides. The oxides are particularly preferred for the reason discussed above. I have also found that these materials, particularly the oxides, can be compounded into very concentrated hydrocarbon comixtures to produce fluid suspensions containing up to 75 weight percent of the metal oxide. I have also discovered that such high concentrations, e.g., above 30 weight percent of the metal oxide, particularly the oxides of zinc and iron, can be obtained only under anhydrous conditions. Accordingly, oxides containing water should be heated to a temperature sufficient to drive off water, e.g., at least about 110.degree., preferably at least about 120.degree. C., for a period sufficient to produce an anhydrous powder. Care should also be taken to assure that the oil is substantially anhydrous so that the total composition, i.e., metal oxide and oil, contains less than 0.5 weight percent water. Under these conditions, relatively stable nutrient-hydrocarbon suspensions can be obtained with only minor agitation and can be maintained in that form with only periodic agitation.
This characteristic greatly facilitates nutrient-sulfur mixing in either batch or continuous operations. It allows for metering of the hydrocarbon-nutrient suspension directly into the melt at a controlled rate or in otherwise controlled amounts to provide a composition having the desired nutrient content. Furthermore, the availability of high nutrient content suspensions also minimizes the amount of hydrocarbon required to transport the desired amount of nutrient in a fluid form, when reduced hydrocarbon concentrations are desired. Similar suspensions can be obtained with the calcium phosphate sources referred to above.
The several components should be mixed in proportions sufficient to assure a product fusion containing at least about 50, preferably at least about 70, weight percent rhombic sulfur. The remainder of the product can comprise hydrocarbon and nutrient compound and/or other solid components such as fillers, clays and the like, as desired. These compositions can contain 1 to about 50, preferably 1 to about 20, weight percent of the hydrocarbon-nutrient comixture.
The hydrocarbon-nutrient suspension can contain as little as one percent nutrient but usually contains at least about 5, often at least about 10, and preferably about 10 to about 75 weight percent nutrient compound.
Better nutrient-hydrocarbon distributions, more stable comixture suspensions, and better product homogeneity are obtained when the nutrient compounds are added as finely divided particles. These are usually characterized as particles passing 50 mesh and preferably passing 100 U.S. Standard screen.
The hydrocarbon-nutrient combination can be added either as a nutrient-hydrocarbon suspension or as the finely divided solids containing sufficient hydrocarbon to completely coat all particle surfaces. Thus, the comixture should contain at least about 2 weight percent, preferably 30 to 95 weight percent hydrocarbon. The fluid suspensions containing at least about 30 weight percent hydrocarbon are presently preferred, particularly in continuous processes.
Due to the characteristics of these hydrocarbon-nutrient compound comixtures, they can be evenly distributed throughout the sulfur melt to form a homogeneous distribution under very low shear conditions, at least as compared to the shear rates required to obtain homogeneity by other procedures. For instance, homogeneous compositions can be obtained at the shear rate existing in turbulent pipeline flow within a reasonable period of time, i.e., within 10 seconds or less. Thus, adequate mixing can be obtained at shear rates corresponding to a linear flow velocity only 10 percent above the maximum laminar flow velocity in a pipeline. The optimum amount of shear and time required for any operation can be easily determined by testing the design composition at several shear rates and times, and selecting the best combination by interpolation.
The nutrient-hydrocarbon comixture can be added to the sulfur melt at a temperature of 120.degree. to about 400.degree. C., preferably 120.degree. to about 250.degree. C. by any one of several well-known procedures. However, in both batch and continuous operations it is presently preferred to meter a fluid suspension of the comixture into the sulfur melt. This procedure facilitates more accurate composition control and leads to homogeneous products with only minor agitation of the sulfur-comix combination. It is particularly adaptable to continuous operations in which the comixture and sulfur melt are continuously blended such as in an in-line mixer. Thus the sulfur melt can be continuously transferred from a sulfur melt reservoir or other container and passed by pumps or under pressure into admixture with the nutrient-hydrocarbon comixture which itself is continuously metered from a comix container. The container is preferably provided with agitation means for maintaining an even suspension of the nutrient compound and hydrocarbon prior to combination with the sulfur melt. These two streams are continuously mixed by any one of numerous known in-line mixers, surge tanks, or the like. The combination is then cooled or quenched to form the homogeneous fusion.
The hydrocarbons are preferably liquid at ambient conditions or, more appropriately, the temperature at which the nutrient compound-hydrocarbon comixture is formed. If this temperature is elevated it is essential only that the hydrocarbon be fluid at that temperature to allow adequate coating and mixing of the two components. The hydrocarbons should have a boiling point below the temperature at which the comixture is introduced to the sulfur melt to avoid flashing and hydrocarbon vapor evolution in the mixing apparatus.
Suitable hydrocarbons include virgin or partially refined crudes or synthetic crudes, e.g., derived from coal, oil shale or other origins of natural or synthetic paraffins, aromatics and/or alkyl aromatics. Illustrative are paraffin waxes, gas oils, crude oils, reduced crude oil residuum, naphtha, diesel oil, fuel oil, light and heavy gas oils, kerosene, jet fuel, 80 to 300 neutral oils, paraffin waxes, hydrocarbon homo- or hetero-polymer oils, waxes or thermoplastics such as polyolefins, polystyrene and the like.
The hydrocarbons should be non-polar and non-reactive with sulfur or other components of the composition at melt temperatures. They are preferably paraffinic, aromatic or alkyl aromatic, or combinations of these. From the standpoint of reactivity and toxicity to both plants and sulfur-active bacteria, the hydrocarbons preferably contain, at most, only minor amounts of olefins, alkynes, alkenyl aromatics or compounds containing reactive or toxic functional groups such as hydroxyl, amino, ether, aldo, keto or carboxyl groups, or the like. However, this exclusion does not apply to most halogenated hydrocarbons which are generally unreactive, at least at the lower melt temperatures within the above ranges. Aromatics are somewhat more refractive to sulfur-active bacteria than are paraffinic hydrocarbons. Accordingly, paraffins or compositions consisting primarily of paraffins are particularly preferred for agronomic use.
These methods do not require surfactants to obtain homogeneous distribution of the comixture in the sulfur melt. In fact, such surfactants are preferably avoided at least in most applications due to their reactivity at melt temperatures and/or their toxicity or refractiveness to sulfur-active bacteria.
Moreover, surfactants would be largely wasted in the preferred particle-forming techniques which involve quenching and subdividing the sulfur-comix blend by contacting with liquid water under high shear conditions. At least some of the surfactant would be abstracted from the sulfur phase under these conditions and surfactant removal unavoidably results in hydrocarbon leaching from the sulfur matrix.
The melt can be cooled and solidified, and, if desired, can be comminuted by any one of several procedures. I have found that these melt blends have certain advantages over other formulations in water quenching systems. Very little, if any, hydrocarbon or nutrient is lost to the water phase. Thus the melt can be cooled into blocks and crushed to the desired particle size or it can be air cooled by conventional methods, e.g., by prilling.
Particularly preferred methods involve water quenching by any one of several techniques. The melt can be sprayed into a standing or agitated aqueous quench in which case particle size can be regulated by spray size and agitation severity. Other methods involve pouring a melt into an agitated aqueous quench, in which case particle size is determined primarily by agitation severity.
A particularly preferred method is disclosed in my U.S. Pat. Nos. 3,637,351, 3,769,378 and 3,830,631, incorporated herein by reference. Briefly, these methods involve contacting a high velocity water spray with a high velocity spray of the homogeneous sulfur-hydrocarbon melt to form a highly turbulent intersection zone of the two sprays in which the melt is simultaneously subdivided and quenched into the porous particles similar to those described in the noted patents.
Whichever method is used, it is often desirable to obtain particles having diameters of about one inch or less, usually about one-half inch or less. The methods of my abovementioned U.S. Patents can produce particles having diameters of about 0.02 to about 0.11 inch, and bulk densities below about 1.9, generally below about 1.3, preferably about 0.9 to about 1.3 grams per cc. They are further characterized by porosities of at least about 0.04, generally about 0.04 to about 0.15 cc's per gram, and internal surface areas of at least about 20, preferably between about 30 and about 100 square meters per gram.